One of conventional functions of a disk array apparatus is a so-called snapshot function that retains an image of a logical volume (hereinafter referred to as the primary volume) at a point in time designated when receiving an instruction to obtain a snapshot. When data is lost due to a human-caused mistake or a state of a file system at a desired point in time needs to be restored, the snapshot function is used to restore the primary volume at that point in time.
If a snapshot is obtained by the snapshot function, data of the primary volume before being overwritten (hereinafter referred to as the old data) is then saved to a logical volume for saving the old data when data is written to the primary volume. Specifically speaking, the difference between the primary volume at the point in time when receiving an instruction to obtain the snapshot, and the present primary volume is saved as the old data to the old-data-saving logical volume. Accordingly, a data image (snapshot) of the primary volume at the point in time when obtaining the snapshot can be restored by combining data stored in the present primary volume and the old data stored in the old-data-saving logical volume.
Such a snapshot function has the advantage of being capable of restoring the primary volume at the point in time when receiving the instruction to obtain the snapshot, by using a smaller storage capacity than a case where the content of the entire primary volume is stored as it is.